Conventional combustion-powered fastener-driving tools normally rely upon the vacuum or reduced pressure conditions, effectively formed within the combustion chamber as a result of the residual combustion gases within the combustion chamber undergoing cooling after the piston has been driven downwardly by the forces generated within the combustion chamber so as to drive a fastener into a substrate, to effectuate the return of the piston back to its original or uppermost position. More particularly, this occurs in view of the fact that such conventional tools normally utilize a plurality of exhaust ports which are disposed in a predetermined array defined within lower side wall portions of the cylinder housing at positions which will be adjacent to the piston when the piston reaches the end of its down stroke or power stroke so as to be disposed at its lowermost position and thereby drive a fastener into a substrate. It can therefore be appreciated that the exhaust ports will be disposed beneath the piston as the piston begins its down stroke or power stroke, however, when the piston reaches the end of its down stroke or power stroke and is disposed at its lowermost position, the piston will effectively pass below the array of exhaust ports such that the combustion chamber is now fluidically connected to the exhaust ports whereby the exhaust gases within the combustion chamber can be discharged or exhausted outwardly to atmosphere from the combustion chamber and the tool. Accordingly, the mass of the gases remaining in the combustion chamber is reduced, such gases will subsequently be cooled and effectively condensed, and the subsequent drop in pressure, relative to the ambient pressure upon the underside of the piston, effectively results in the formation of vacuum or reduced pressure conditions within the combustion chamber above the piston, thereby effectively drawing the piston back to its original or uppermost position.
The problem with such a system is that when the piston reaches the end of its down stroke or power stroke so as to be disposed at its lowermost position, the piston will normally encounter a bumper which effectively controls the deceleration and travel length of the piston. Accordingly, the piston will effectively bounce off or back from the bumper thereby covering or closing off the exhaust ports before a sufficient amount of the combustion gases, disposed within the combustion chamber, can be exhausted to atmosphere. The piston, now moving in the upward direction, therefore compresses the combustion gases which are disposed above it and effectively trapped within the combustion chamber until the upward movement or energy of the piston is effectively dissipated or exhausted as a result of such gas compression. In addition, the compressed gases will subsequently expand and tend to move the piston back downwardly so as to effectively return the piston toward its lowermost position. This phenomenon can cause a double strike which might tend to partially drive another fastener out from the tool. Alternatively, the piston can oscillate for a number of cycles causing fresh or ambient air, disposed beneath the piston, to effectively short circuit around the piston, by means of the exhaust ports defined within the side wall portions of the cylinder housing, whereby the vacuum or low pressure conditions within the combustion chamber will effectively be reduced thereby causing the piston to be returned slowly to its original or uppermost position, or alternatively, the piston may only achieve a partial or incomplete return movement. Still further, since this process has effectively caused hot combustion gases to be maintained within the tool for an abnormally long period of time, the tool will be prone to overheating.
A need therefore exists in the art for an improved combustion gas exhaust and piston return system whereby the aforenoted problems will not occur within the combustion-powered fastener-driving tool.